Optical gratings are useful in controlling the paths or properties of traveling light. Gratings based on optical fibers are of particular interest as components in modern telecommunication systems. Basically, optical fibers are thin strands of glass capable of transmitting information-containing optical signals over long distances with low loss. In essence, an optical fiber is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core. Typical optical fibers are made of high purity silica, and various concentrations of dopants may be added to control the index of refraction.
Optical gratings are important elements for selectively controlling specific wavelengths of light transmitted within optical systems such as optical communication systems. Such gratings may include Bragg gratings, long-period gratings, and diffraction gratings. These gratings typically comprise a body of material with a plurality of spaced-apart optical grating elements disposed in the material. Often, the grating elements comprise substantially equally-spaced index perturbations, slits, or grooves. For all types of gratings, it would be highly useful to be able to reconfigure or tune the grating to selectively adjust the controlled wavelengths. As an illustration, the Bragg grating, long-period grating, and diffraction grating are discussed below.
A typical Bragg grating comprises a length of optical waveguide, such as optical fiber, in which a plurality of perturbations in the index of refraction are substantially equally-spaced along the waveguide length. These perturbations selectively reflect light of wavelength .lambda. equal to twice the spacing .LAMBDA. between successive perturbations times the effective refractive index. In other words, .lambda.=2n.sub.eff .LAMBDA., where .lambda. is the vacuum wavelength and n.sub.eff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Bragg gratings have found use in a variety of applications including filtering, adding and dropping signal channels, stabilizing semiconductor lasers, reflecting fiber amplifier pump energy, and compensating for waveguide dispersion.
Bragg gratings may be conveniently fabricated by doping a waveguide core with one or more dopants sensitive to ultraviolet light, e.g., germanium or phosphorous, and exposing the waveguide at spatially periodic intervals to a high intensity ultraviolet light source, e.g., an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce long-term perturbations in the local index of refraction. The appropriate periodic spacing of perturbations can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
A difficulty with conventional Bragg gratings, however, is that they filter light of only a fixed wavelength. Each grating selectively reflects light in a narrow bandwidth centered around .lambda.=2n.sub.eff .LAMBDA.. However, in many applications, such as wavelength division multiplexing (WDM), it would be desirable to have a grating whose wavelength response can be controllably altered.
One attempt to make a tunable waveguide grating involves applying strain to the grating using a piezoelectric element. See Quetel et al., 1996 Technical Digest Series, Conf. on Optical Fiber Communication, San Jose, Calif., Feb. 25-Mar. 1, 1996, Vol. 2, p. 120, paper No. WF6. A difficulty with this approach is that the strain produced by piezoelectric actuation is relatively small which limits the tuning range of the device. Moreover, this approach requires that electrical power be continuously applied at relatively high voltage, e.g., approximately 100 volts. Other tunable gratings involving the application of strain to the grating are disclosed in U.S. patent application Ser. No. 08/791,081 filed by Jin et al on Jan. 29, 1997, U.S. patent application Ser. No. 09/020,206, filed by Espindola el al on Feb. 6, 1996, U.S. patent application Ser. No. 08/971,956 filed by Jin el al on Oct. 27, 1997, and U.S. patent application Ser. No. 08/971,953 filed by Fleming et al. on Oct. 27, 1997, all of which were assigned to the present assignee and are incorporated herein by reference.
Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with low back reflections. A long-period grating typically comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced along the waveguide by a periodic distance .LAMBDA.' which is large compared to the wavelength .lambda. of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing .LAMBDA.' which is typically at least 10 times larger than the transmitted wavelength, i.e. .LAMBDA.'.gtoreq.10.lambda.. Typically .LAMBDA.' is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5.LAMBDA.' to 4/5.LAMBDA.'. In some applications, such as chirped gratings, the spacing .LAMBDA.' can vary along the length of the grating.
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with conventional Bragg gratings in which light is reflected and stays in the waveguide core, long-period gratings remove light without reflection, such as by converting it from a guided mode to a non-guided mode. A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. Often, it is a cladding mode. The spacing .LAMBDA.' of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength .lambda..sub.p from a guided mode into a non-guided mode, thereby reducing in intensity a band of light centered about the peak wavelength .lambda..sub.p. Alternatively, the spacing .LAMBDA.' can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is substantially stripped off the fiber to provide a wavelength dependent loss. Such devices are particularly useful for equalizing amplifier gain at different wavelengths of an optical communications system.
A difficulty with conventional long-period gratings, however, is that their ability to dynamically equalize amplifier gain is limited, because they filter only a fixed wavelength acting as wavelength-dependent loss elements. Each long-period grating with a given periodicity (.LAMBDA.') selectively filters light in a narrow bandwidth centered around the peak wavelength of coupling, .lambda..sub.p. This wavelength is determined by .lambda..sub.p =(n.sub.g -n.sub.ng).multidot..LAMBDA.', where n.sub.g and n.sub.ng are the effective indices of the core and the cladding modes, respectively. The value of n.sub.g depends on the core and cladding refractive indices while the value of n.sub.ng depends on the core, cladding and air indices.
In the future, multi-wavelength communication systems will require reconfiguration and reallocation of wavelengths among the various nodes of a network depending on user requirements, e.g., with programmable add/drop elements. This reconfiguration will impact upon the gain of the optical amplifier. As the number of channels passing through the amplifier changes, the amplifier will start showing deleterious peaks in its gain spectrum, requiring modification of the long-period grating used to flatten the amplifier. Modifying the long-period grating implies altering either the center wavelength of the transmission spectrum or the depth of the coupling.
Thus, there is a need for a long-period grating whose transmission spectra can be controlled as a function of the number of channels and power levels transmitted through an amplifier. It is desirable to have a long-period grating which, upon activation, can be made to dynamically filter other wavelengths (i.e., besides .lambda..sub.p). It is also desirable to be able to selectively filter a broad range of wavelengths. Further, a tunable long period grating would be useful for suppressing amplifier spontaneous emission (ASE), and for use as a tunable loss element for filtering out undesirable remnant signals from communication channel add/drop operations.
Diffraction gratings typically comprise reflective surfaces containing a large number of parallel etched lines of substantially equal spacing. Light reflected from the grating at a given angle will have a different spectral content depending on the spacing. The spacing in conventional diffraction gratings, and hence the spectral content, is generally fixed.
As may be appreciated, those concerned with technologies involving optical communications systems continually search for new designs and methods for making tunable optical grating devices. It should be apparent from the foregoing that there remains a need for a tunable optical grating device which can include a Bragg grating, a long-period grating, or a diffraction grating that does not require a continuous application of power. For certain optical networking applications in telecommunications systems, it would be advantageous to shift the wavelength with accuracy, e.g., by about one-half or one channel width. This invention discloses such a digitally tunable (switchable) grating device and telecommunication systems comprising the inventive device.